Ball grid arrays are arrays for receiving solder balls which are used as electrodes for connection to integrated circuits. They are placed whilst the integrated circuit is still within a substrate prior to or after dicing of said substrate.
The substrate, in general terms, is passed into a flux deposition platform and then to a ball mounting platform where upon the balls are mounted within the flux in precisely located arrangements.
Solder Ball Placement Machines are employed to attach the solder balls within the area-array package substrates where they form the final interconnection to the substrate. The Solder Ball Placement Machine performs two main processes, which are flux deposit on the substrate, followed by solder balls placement on the substrate. Flux is used to remove oxidation on the solder pad for better connectivity and bonding, and to hold the solder ball in place before the substrate is sent to the reflow oven to melt the solder ball to complete the bonding.
In one method, the flux and solder balls processing regions are located in series whereby the substrate is passed within the flux processing region so that flux is applied to the array. Subsequently the substrate is passed to the ball mounting platform whereby a ball robot places the solder balls in a precise benchmark. The substrate is subsequently passed out of the ball mounting apparatus and subsequently sent for dicing.
For this arrangement, fiducial vision of the substrate on each platform is taken so as to locate the position and orientation of the substrate and so precisely place both the flux and solder balls.
A flaw in this method, however, is the need to take two separate set of fiducial vision of the substrate on the two separate platforms. This will worsen the bottle neck at the ball mounting station as the ball pick head being the apparatus for placing the balls must first determined whether the head is fully laden with balls such that there are none missing from the cavity used for placing the balls and subsequently checked that all balls were deposited from the head and no particular cavity still having a ball jammed within the head and so not transferred to the array. This double inspection system controls the flow of substrates through the ball mounting device and can consequently miss the overall speed of this process.
In a different arrangement, the flux and ball mounting regions are placed parallel to each other such that the substrate is passed into the device and the flux positioning head and ball pick head positioned to access the substrate without having to move the substrate from one station to the next. In this way, the fiducial vision of the substrate to ensure orientation and position of the substrate is correct, need only be taken once and not separately at the flux region and ball region, so consequently for this reason will be a faster process compared to the previous. However, the limitation of this arrangement is that the single platform can only be worked on by the Flux or the Ball Head at one time and either one will be idle, so the benefit of a single fiducial vision check is wasted as the bottle neck is now at the single mounting platform and it causes an increase in the machine cycle time.
In another facet of the process, the flux pool generally comprises a pair of sweeping flux applicators mounted on a linear slide and arranged to travel backward and forward over a supply of flux deposited on a plate. The flux applicator, during the sweeping motion forms a layer of flux at a pre-determined thickness as controlled by the height of the flux applicator. The sweeping action is driven by a speed control motor and often including a pair of pneumatic actuators.
During sweeping, one flux applicator will be in a lowered position at the fixed height from the flux pool to form the predetermined flux thickness. The thickness of flux on the plate will control the amount of flux taken by a flux tool which then applies the controlled amount of flux to a substrate. As a means of ensuring the flux pool has a sufficient supply, there is a sensor which measures the level of flux in the flux pool that on reaching a lower limit, which activates a flux refiner to add new flux to the flux pool.
The flux refiner typically comprises a syringe subjected to an external air supply so as to apply pressure to the plunger of the syringe and consequently inject the flux into the flux pool.
Whilst there are variations known in the industry many typically have similar arrangements including the use of a syringe in order to inject flux into the flux pool. Accordingly, these suffer the problem of syringes generally in that not all the contents of the syringe can be completely removed. Instead a “slug” or “tailing” of the flux remains in the exit chamber preceding the nozzle of the syringe. This remaining flux may be partially exposed to the external environment and therefore, can become contaminated on the next usage of the syringe. Further, whilst the flux is viscous, it can still drip from the nozzle and so the tailing remaining in the nozzle may drip and so place flux on the peripheral elements of the flux pool. Accordingly the flux that contacts areas outside the flux pool may further become contaminated and if then inadvertently added to the flux pool can cause contamination to the entire flux pool.
This may also result in contamination that may spread to subsequent substrates, contaminating an entire batch of substrates to which the solder balls are being attached.
In a further facet, once the flux is applied to the substrate, solder balls are then mounted to the flux for further processing. The arrangement of the solder balls on to the substrate requires a very precise arrangement to a very high tolerance.
Templates are used as a means of establishing a predetermined array for the batch process of placing small scale units such as solder balls for down stream manufacturing purposes.
The time consuming activity of placing the solder balls within the highly defined arrays is one that can be alleviated by automating their placement of the units within said arrays. One such method involves pouring the solder balls into the templates for a gravity placement of the arrays. It follows that this method would be extremely inaccurate. Another such method involves sweeping a reservoir of solder balls across the template so as to maintain a more precise correspondence with the array through maintaining a pre-determined gap between the reservoir and the array. The maintenance of this very precise gap has led to a high quality in output and less wastage through misplacement of solder balls within the array.
The difficulty with this approach is the level of tolerance required to maintain the gap. It is necessary to establish a datum from which to measure the gap and for the mechanical system required for distribution of the solder balls to the array. Fixed points relative to the reservoir such as a base plate underlying the template or a linear slide to which the reservoir is mounted so as to distribute the balls are logical places to establish the datum. The precision requires a high degree of machining which adds to the manufacturing costs of the device. Further poor linearity of the linear slide or warpage of the base plate must be avoided to maintain the precision required.
Without maintaining a high precision for the gap, it is very difficult to accurately distribute the solder balls particularly as a template may require an array having a planar area of 300 mm×90 mm. The degree of flatness required over such an area represents a significant manufacturing cost for the equipment.
Further, even if the machining is maintained to a very high tolerance, the installation of various components including the template will also require a high degree of quality. If the template is not precisely fitted to the device, then even a manufacture of high tolerance will not overcome the shortcomings of a poor installation.